Terrorweb Tunnel is a tunnel connecting the Thondroril River and Plaguewood. That's the northwest corner of the Eastern Plaguelands.
I first heard of this tunnel when Naxxramas came out. Not the one over Dragonblight. The one over the Plaguelands. The one that barely anyone saw and had an expensive attunement unless you did a long rep grind. I think it was 1.9.
With the addition of Naxxramas came a few token-purchasable items from the Argent Dawn. These tokens came from turning in certain mats, including spider bits. So the first time I heard of thus tunnel was in regard to Naxxramas farming.
Long story short: I thought Terrorweb Tunnel was part of Naxxramas. This led to much confusion when I went in there at 70. Where was the tunnel!?
What mixups of geography baffled you?
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